Such a headset may be used for listening to an audio source (e.g. music) coming from an appliance such as an MP3 player, a radio, a smart phone, etc., to which it is connected via a wired connection or indeed via a wireless connection, in particular of the Bluetooth type (registered trademark of Bluetooth SIG).
If it is provided with a microphone set suitable for picking up the voice of the wearer of the headset, the headset may also be used for communications functions, such as “hands-free” telephony functions, in addition to listening to the audio source. The transducer of the headset then reproduces the voice of the remote speaker with whom the wearer of the headset is in conversation.
The headset has two earpieces connected together by a headband. Each earpiece comprises a closed shell housing a sound playback transducer (referred to more simply herein as a “transducer”) and being designed to be pressed around the user's ear with an ear-surrounding cushion being interposed to isolate the ear from the external sound environment.
When the headset is used in a noisy environment (metro, busy street, train, airplane, etc.) the wearer is protected from the noise in part by the earpieces of the headset, since they provide insulation by virtue of the closed shell and the ear-surrounding cushion.
Nevertheless, that purely-passive protection is partial only, and a fraction of the external sound, in particular in the low portion of the frequency spectrum can reach the ears by passing through the shells of the earpieces, or indeed through the wearer's skull.
That is why so-called active noise control (ANC) techniques have been developed that are based on the principle of picking up the incident noise component by means of a microphone placed on the shells of the earpieces of the headset, and on superposing an acoustic wave in time and in three dimensions on said noise component, which acoustic wave is ideally an inverted copy of the pressure wave of the noise component. The idea is to create destructive interference with the noise component, thereby reducing and ideally canceling the pressure variations of the interfering acoustic wave.
Implementing this principle involves overcoming a large number of difficulties that have lead to a wide variety of proposals, which proposals can be grouped into two categories.
A first category is that of ANC methods using adaptive filters, i.e. filters having a transfer function that is modified dynamically and continuously by an algorithm that acts in real time to analyze the signal. Such processing is made possible in particular as a result of developments in techniques for digitizing and processing signals by means of specialized processors, that are programmed to implement algorithms in real time.
DE 37 33 132 A1 is a typical example of ANC processing making use of such adaptive filters. Other examples of ANC methods involving adaptive filters are described in particular in U.S. Pat. No. 6,041,126 A, US 2003/0228019 A1, and WO 2005/112849 A2.
Those techniques can be effective in terms of reducing noise, but they present the drawback of necessarily being digital and of requiring relatively large amounts of computation power, with the consequences of being relatively complex to design and quite expensive to make.
Furthermore, the digital processing gives rise to non-negligible delays in the compensation signal, and the adaptive feature involves some minimum length of convergence time for the algorithms. All of that is harmful to the reactivity of the system, in particular in response to noises that are irregular. As a result, the denoising is effective in particular against noise that is essentially periodic and in narrow band.
The second category of ANC methods—to which the technique of the invention belongs—is that of filter systems that are static, i.e. non-adaptive, in which the parameters of the various filters used are predetermined.
Such ANC systems combine static filtering of the feedback type in a closed loop and of the feedforward type in an open loop. The feedback channel is based on a signal picked up by a microphone placed inside the acoustic cavity (referred to below as the “front” cavity) defined by the shell of the earpiece, the ear-surrounding cushion, and the transducer. In other words, this microphone is placed close to the user's ear and receives mainly the signal produced by the transducer and the residual, non-canceled noise signal that is still perceptible in the front cavity. The audio signal from the music source that is to be played back by the transducer is subtracted from the signal from this microphone so as to constitute an error signal for the feedback loop of the ANC system. The feedforward filter channel makes use of the signal picked up by the external microphone picking up the interfering noise that exists in the immediate environment of the wearer of the headset.
One such system is described in particular in US 2010/0272276 A1 that, in addition to the feedback and feedforward filter channels, also provides a third filter channel that processes the audio signal from the music source that is to be played back. The output signals from the three filter channels are combined and applied to the transducer in order to play back the signal from the music source in association with a signal for suppressing the surrounding noise.
Since the parameters of the various filters are static, static filtering techniques can be implemented equally well in analog technology or in digital technology, in manners that require fewer resources than are required for adaptive filter techniques.
Nevertheless, static filtering methods present limitations and drawbacks.
A first drawback is relatively great sensitivity to variations in the electroacoustic paths between the transducer and the error microphone, i.e. the internal microphone placed in the front cavity. The electroacoustic response between those two elements can be modified as a result of the variations in the volume of the front cavity and in its sealing relative to the outside. The main factors that might cause this electroacoustic response to vary are the positioning of the headset on the head, the shape of the user's ear, the tightness with which the headset is pressed against the head, and the presence of hair where the ear-surrounding cushions press against the head. Other variations may be due to the electronic components used (resistors, capacitors, transducer, and microphone) since they present electrical characteristics that might fluctuate over time.
These variations in the acoustic response can produce an undesirable effect known as the “waterbed” effect: beyond the main noise suppression frequency band, noise becomes amplified in a relatively narrow frequency band, generally around 1 kilohertz (kHz) in a manner that is entirely perceptible and naturally unwanted. If this phenomenon is too great it can even give rise to a Larsen effect, a phenomenon that is to be observed with many headsets when the cushion is accidentally removed.
Another factor to be taken into account is the volume of the front cavity, insofar as a small front volume increases the variability of the electroacoustic response between the transducer and the error microphone, since under such circumstances, there is a greater relative variation in volume between the normal listening position and the transition position in which the user moves the headset closer to the head.
A small volume for the front cavity is thus an additional factor for loss of stability in the feedback loop, with the same consequences as those explained above. In practice, it is desirable for earpieces to be made with relatively small volume, both for reasons of comfort and for reasons of weight, and that goes against the requirement for stability in the ANC system.
Specifically, the various filtering channels are adjusted so as to produce performance that corresponds to a given electroacoustic response, with gain and phase margins that make it possible to guarantee stability that is sufficient and performance that is maximized. In this respect, it is considered that a closed-loop system must generally present a phase margin of more than 45° and a gain margin of at least 10 decibels (dB). However those theoretical margins are often found to be insufficient because of the great variability in the electroacoustic responses that are to be found in practice in the field of headsets with active noise control.